Conventional methods for the manufacture of polypropylene generally involve stereospecific polymerization of the propylene monomer in the presence of a catalyst containing a coordination complex of a transition metal halide and an aluminum alkyl compound or similar organometallic compound. The product from this reaction is a slurry of polypropylene and catalyst in a hydrocarbon diluent. A low molecular weight alcohol is normally added to this slurry to quench the reaction and solubilize the catalyst in a step sometimes referred to as the kill step. Amorphous and low molecular weight polypropylene remain in solution in the diluent as a by-product. Subsequent steps of the process generally involve recovery of the crystalline polypropylene, neutralization of the active catalyst, and recovery of the solution containing some of the alcohol and hydrocarbon diluent (spent polymerization medium). The crystalline polypropylene is normally recovered as a high purity polymer suitable for use in a wide variety of coating, casting and molding applications. The soluble polypropylene can be recovered from the spent polymerization medium, after some purification, as an odorless, off-white, tacky thermoplastic material. Although at one time this material was of no commercial importance, it is now used as a hot-melt, adhesive compounding agent.
Existing procedures for separating the soluble polypropylene, for example as set out in U.S. Pat. Nos. 3,296,240 and 3,437,645, involve partially steam stripping the spent polymerization medium at elevated temperatures. Surfactants are sometimes added to aid formation of an isoluble crumb. This solid crumb can then be removed from the upper portion of the liquid and discarded or further purified for commercial use. However, the crumb takes with it five to ten percent of the polymerization medium and larger amounts of the water and hence is unsuitable as removed for uses which require medium-free polymer. Additionally, the process is wasteful of the polymerization medium and the large volumes of water used are contaminated with relatively high levels of suspended solids, surfactants, polymerization media and other organic components which make the water difficult to treat by conventional waste water reclamation methods. In short, existing processes are messy, wasteful and produce unacceptably large volumes of contaminated waste water in reclaiming an amorphous polypropylene of low purity.
A method of separating low molecular weight, i.e. M.sub.n of about 1,400, synthetic waxes made using Ziegler catalysts, metal oxide catalysts etc., e.g. waxy polyethylenes, is set out in U.S. Pat. No. 3,775,484. The process involves essentially complete stripping of the light ends from the wax to produce a wax layer over an aqueous layer, decanting the wax layer, and gas drying the decanted product.
Now a process has been found by which relatively pure soluble polypropylene can be reclaimed in an economical fashion in which the problems of contaminated waste water and polymerization diluent losses are greatly reduced.